Not Applicable.
1. Field of the Invention
The present invention is directed generally to an apparatus and method for measuring quantities of a mixture and, more particularly, to an apparatus and method for measuring the enthalpy and flow rate of a mixture.
2. Description of the Background
Enthalpy and flow rate are features that may be used to characterize a steam-water mixture flowing in a restricted area, such as a conduit. The change in enthalpy of a system is the heat transferred from surroundings to system in a constant pressure process. The flow rate is the amount of flow of the mixture through a given volume per unit time. It is desirable to measure both the enthalpy and the flow rate of steam-water mixtures flowing in a pipeline, due to the fact that these measures can be used to define the desirable characteristics of the two-phase mixture as to a particular application. For example, two-phase mixtures of steam and water are produced from geothermal wells. This two-phase flow may proceed to a power plant, for example, and, at the power plant, the steam is preferably separated from the water, and the steam proceeds through a turbine in order to generate electricity. Thus, it is highly desirable to measure the characteristic measures of the two phases in order to assess the quality and quantity of electricity generation.
Two-phase flow measurement using a differential pressure in a nozzle is known in the art. However, the accuracy of the flow measurement in existent methodologies is often compromised due to a lack of a continuous measurement of enthalpy. Currently, water and steam fraction are measured periodically by taking a well offline and directing the flow to a test separator. Thus, the enthalpy and flow rate of a well are measured non-continuously, and certain measurements are made off-line.
It is known in the art to make use of devices that measure water fraction in a pipeline using a measurement of the capacitance of the two-phase flow. Certain of these known methods might also be used to measure resistivity of the two-phase flow. However, these known methods make use of delicate instruments that impede the flow, and these delicate instruments are not generally well-suited for the often severe environment of use, such as use in a well field. Further, the use of these delicate instruments is often disadvantageous because the instruments make use of only two electrodes, and the two electrodes not only do not accurately measure flow rate and enthalpy, but may further be rendered inaccurate by mineral deposits.
Therefore, the need exists for an apparatus and method that provides accurate measurement of the flow rate and enthalpy of a two-phase flow, continuously and on-line.
The present invention relates to a measurement device for measuring a multi-phase mixture in a restricted area. The device may include two current electrodes within a restricted area, wherein a first electrical current is excited within the restricted area between said two current electrodes, at least two voltage electrodes within the restricted area, wherein a voltage is present between at least two of said at least two voltage electrodes, and wherein the voltage is produced by the first electrical current, and at least one meter, wherein a multi-phase characteristic of the multi-phase mixture may be determined by the at least one meter in accordance with a resistivity generated by the at least one voltage and the first electrical current, and/or in accordance with a flow rate of the liquid mixture.
The present invention additionally relates to a measurement system for measuring a multi-phase mixture in a restricted area. The system includes a restricted area, two current electrodes within the restricted area, wherein a first electrical current is excited within the restricted area between the two current electrodes, at least two voltage electrodes within the restricted area, wherein a voltage is measured between at least two of the at least two voltage electrodes, and wherein the voltage is produced by the current, at least one measuring device, wherein a multi-phase enthalpy of the multi-phase mixture is determined by the at least one measuring device in accordance with a resistivity generated by the at least one voltage and the first electrical current, and wherein a multi-phase flow rate is determined by the at least one measuring device.
The present invention relates to a method of measuring a multi-phase mixture in a restricted area. The method includes exciting a first electrical current between two current electrodes within the restricted area, measuring a voltage between at least two voltage electrodes within the restricted area, wherein the voltage is produced by the current, and measuring a multi-phase enthalpy of the multi-phase mixture according to a resistivity measure generated by the voltage and the current.
The present invention determines enthalpy of a steam-water mixture flowing in a restricted area by measuring resistivity, and preferably determines flow rate from measurements of pressure. The restricted area preferably includes a nozzle that agitates the flow so that the phases are mixed and evenly dispersed where the resistivity, enthalpy, and/or flow measurements are made. This mixing in the nozzle allows for a measurement of flow rate due to the decrease in pressure in the nozzle, and the mixing improves the resistivity measurement by improving the evenness of a mixture that may include an element of approximately infinite resistivity.
The present invention solves problems experienced in the prior art because it provides an apparatus and method that provides measurement of the flow rate and enthalpy of a two-phase flow, continuously and on-line. Those and other advantages and benefits of the present invention will become apparent from the detailed description of the invention hereinbelow.